Copepods are a group of small crustaceans found in the sea and nearly every freshwater habitat. According to the classification system of Martin and Davies (2001), the copepods form a subclass belonging to the subphylum Crustacea (crustaceans). Subphylum Crustacea is a large group of the phylum Arthropoda, comprising almost 52,000 described species. Six classes of the Crustaceans are usually recognized. Subclass Copepoda of the class Maxillopoda comprise ten orders, of which the order Calanoida include 43 families with about 2000 species. Many species are planktonic (drifting in sea waters), but more are benthic (living on the ocean floor), and some continental species may live in limno-terrestrial habitats and other wet terrestrial places, such as swamps, under leaf fall in wet forests, bogs, springs, ephemeral ponds and puddles, damp moss, or water-filled recesses (phytotelmata) of plants such as bromeliads and pitcher plants. Many live underground in marine and freshwater caves, sinkholes, or stream beds.
Two of the most abundant northern calanoid species is C. finmarchicus which is commonly regarded as a northern boreal species inhabiting North Atlantic Ocean, while C. hyperboreus is an arctic species.
The evolution of antibiotic-resistant pathogenic bacteria has stimulated the search for alternative antimicrobial agents from natural sources. Antimicrobial activity has previously been detected in several decapod crustaceans, including lobster, crabs, shrimps and freshwater crayfish. The search for novel compounds displaying antimicrobial activity has led to the identification of several antimicrobial peptides and proteins in decapod crustaceans (Haug et al., 2002).